1. Field of the Invention
This invention relates to esters of oleic acid estolides having saturated fatty acid and groups, and their use as biodegradable base stocks and lubricants.
2. Description of the Prior Art
Synthetic esters, such as polyol esters and adipates, low viscosity poly alpha olefins (PAO), such as PAO 2, vegetable oils, especially Canola oil and oleates are used industrially as biodegradable basestocks to formulate lubricants. Lubricants usually contain 80-100% wt. basestock and 0-20% wt. additives to tailor their viscometric properties, low temperature behavior, oxidative stability, corrosion protection, demulsibility and water rejection, friction coefficients, lubricities, wear protection, air release, color and other properties. Biodegradability cannot be improved by using additives.
In the recent prior art, a fair amount of attention has been given to estolides as having potential for base stocks and lubricants. An estolide is a unique oligomeric fatty acid that contains secondary ester linkages on the alkyl backbone of the molecule.
Estolides have typically been synthesized by the homopolymerization of castor oil fatty acids [Modak et al., JAOCS 42:428 (1965); Neissner et al., Fette Seifen Anstrichm 82:183 (1980)] or 12-hydroxystearic acid [Raynor et al., J. Chromatogr. 505:179 (1990); Delafield et al., J. Bacteriol. 90:1455 (1965) under thermal or acid catalyzed conditions. Yamaguchi et al., [Japanese Patent 213,387, (1990)] recently described a process for enzymatic production of estolides from hydroxy fatty acids (particularly ricinoleic acid) present in castor oil using lipase. Estolides derived from these sources are composed of esters at the 12 carbon of the fatty acids and have a residual hydroxyl group on the estolide backbone. In addition, the level of unsaturation in the produced egtolides (expressed through e.g. iodine value) is not significantly lower than that in raw materials, i.e., hydroxy fatty acids.
Erhan et al. [JAOCS, 70:461 (1993)], reported the production of estolides from unsaturated fatty acids using a high temperature and pressure condensation over clay catalysts. Conversion of the fatty acid double bond into an ester functionality is a strikingly different method than the hydroxy esterification process.
The parent application of Isbell et al., U.S. Ser. No. 09/191,907, now U.S. Pat. No. 6,018,063, issued Jan. 25, 2000, reported a novel class of estolide compounds derived from oleic acids and having superior properties for use as lubricant base stocks. These compounds are characterized by Formula I, infra, wherein the "capping" fatty acid moiety R.sub.3 is typically oleic or stearic acid. Studies with these estolides revealed that the stearic acid has the effect of adversely raising the pour point of the lubricant; and, generally, the greater percentage of stearic acid as the capping moiety, the higher the pour point temperature.